1. Field of the Invention
The present invention is related to optically controlled bulk semiconductor switches and, more particularly, to a switch which does not require light to sustain conduction and is capable of high current gain and high current density.
2. Description of the Related Art
Many varieties of optically controlled semiconductor switches exist. They are commonly used in photodiodes, phototransistors, photothyristors and light sensitive resistors of many types. The most commonly used light-sensitive semiconductor device is the photodiode which is used in many applications, including fiber optic communication systems, etc. The currents handled by such devices are relatively low, typically in the milliampere range. One example is the PIN diode taught by U.S. Pat. No. 4,240,088 to Myers. Other light-activated devices include the thyristors taught by U.S. Pat. Nos. 3,832,732 to Roberts and 4,001,865 to Voss, the microwave switching circuit taught by U.S. Pat. No. 4,396,833 to Pan and many other patents. Some, such as U.S. Pat. Nos. 4,626,883 to Kash et al. and 3,917,943 to Auston, describe switching elements with picosecond switching times.
The Auston '943 patent uses light only to control the state of the switch, i.e., whether the switch is ON or OFF, but unlike a photodiode, does not require light to be directed onto the surface of the semiconductor to sustain conduction. In the Auston patent only a shallow region very close to the surface of the semiconductor becomes conductive. The switch is turned OFF by light, having a wavelength different than the initiating light beam, which grounds the switch by directing the flow to a ground plate on the opposite surface of the semiconductor switch.
Other light activated switches, such as those taught by U.S. Pat. Nos. 4,438,331 to Davis and 4,376,285 to Leonberger et al., are able to carry a larger amount of current by making an entire block of semiconductor material conductive; however, like photodiodes, they require continuous light to sustain conduction. Such switches may be satisfactory when they are required to be only briefly closed, but they require unacceptably large amounts of energy for switches which are designed to remain closed for longer periods of time, particularly in view of their relatively low current gain.
Electron-bombarded semiconductor (EBS) devices are another type of high-speed switching device. However, EBS devices are limited in output power and current density due to the space charge limited current condition.
Higher current densities, up to 10.sup.2 A/cm.sup.2 are provided by diffuse discharge switches, such as the switch taught by U.S. Pat. No. 4,0643,130 to Hunter. However, while diffuse discharge switches can be turned ON in less than a nanosecond, they are relatively slow to turn OFF, taking approximately 100 nanoseconds, have a moderate current gain between 10.sup.2 and 10.sup.3 and like all EBS, require continuous bombardment by an electron beam to sustain conduction.